FIG. 1 shows the structure of an apparatus that performs single wafer processing in a hermetically sealed chamber used in a wafer preprocessing step of semiconductor production. As shown in FIG. 1, the processing apparatus comprises: a wafer placing stand 3 on which a wafer 1, a glass substrate, and other electronic material substrates are placed; an exhaust duct 5 that surrounds the wafer stand 3; a baffle plate that is provided with baffle holes 7 and placed on the exhaust duct 5; and an exhaust pipe 11 that is connected to the exhaust duct 5. In FIG. 1, arrows 13 indicate the exhaust flow of the process gas.
To perform uniform processing on the surface of the wafer 1 in the apparatus for performing single wafer processing in a hermetically sealed chamber, for instance, in a CVD (Chemical Vapor Deposition) apparatus, an etching apparatus, an annealing apparatus, and a dry cleaning apparatus, it is necessary to supply the process gas uniformly on the surface of the wafer 1 with a shower head, and to flow the exhaust gas generated from the contact with the wafer 1 in the radial direction evenly at any circumferential angle of the wafer 1 (this process will be hereinafter referred to as “uniform exhaust”).
The uniform exhaust is essential in the single wafer processing apparatus (such as a metallic CVD apparatus) to perform a uniform film forming operation. The single wafer processing apparatus that performs a so-called rate-determining process in which the reaction rate on the surface of the wafer 1 depends on the transportation rate of the material gas. Also, in a plasma processing apparatus in which the residence time of the gas on the wafer has an influence on the concentration variation of labile species (i.e., in a plasma CVD apparatus or a plasma etching apparatus), the uniform exhaust is essential for obtaining a uniform film forming rate and an etching rate.
In a normal case where the connecting port of the exhaust pipe 11 connected to the pump for exhausting the wafer processing gas deviates from the center axis of the wafer 1 and opens toward the gas processing chamber, it is difficult to exhaust the gas evenly from the center of the gas flow on the wafer in the every circumferential direction. In order to solve this problem, the exhaust duct 5 is extended from the connecting port of the exhaust pipe 11 to the entire periphery of the wafer 1, and the baffle plate 9 is disposed as a separation wall on boundary between the exhaust duct 5 and the chamber. Normally, a number of baffle holes 7 are formed at uniform intervals in the baffle plate 9, thereby obtaining uniform exhaust.
The baffle plate 9 aims to decrease unevenness of exhaust gas flow on the surface of the wafer 1. The principles of the baffle plate 9 reside in forming the baffle holes 7 that causes a greater flow resistance than the flow resistance in the exhaust duct 5 so as to reduce a variation in exhaust conductance that depends on the circumferential angle and to make the flow rate of the exhaust gas around the wafer 1 uniform in the circumferential direction.
To make the exhaust flow rate of the baffle holes 7 uniform, it is necessary to equalize the differential pressures at the respective baffle holes 7 on both sides of the baffle plate 9. However, in a conventional technique of forming the identical baffle holes 7 on the baffle plate 9 at uniform intervals, it is difficult to obtain uniform exhaust flow 13 at every circumferential angle of the wafer 1. The main reason for this is that the difference between the inner pressure of the exhaust duct 5 and the pressure in the chamber in the normal film forming operation is small. As a result, the adverse influence of the inner pressure of the exhaust duct 5 on the variation of the differential pressures due to a fluid pressure loss becomes too great to ignore.
It may be possible to reduce the pore size of each baffle hole 7 so as to increase the differential pressures on both sides of baffle hole 7 to such a degree that can nullify a variation of the inner pressure of the exhaust duct 5. However, a smaller pore size of each baffle hole 7 will results in an increase of the pressure in the chamber. Enlarging the pore size of each baffle hole 7 to reduce the flow resistance in the exhaust duct 5 is also undesirable, because a larger pore size results in a decrease of the flow resistance in the exhaust duct 5. Accordingly with the conventional apparatus employing the baffle plate 9, there remain problems that a relatively large amount of exhaust gas flows on the surface of the wafer in the vicinity of the connecting port of the exhaust pipe 11, and that the flow rate of the exhaust gas at the connecting port of the exhaust pipe 11 and at the baffle holes 7 on the other side is low.
In order to solve the above problems, Japanese Laid-Open Patent Application No. 63-141318 discloses a conductance plate provided with a plurality of holes having pore sizes proportional to the distances from the exhaust port. However, such a conductance plate does not function sufficiently to obtain uniform processing on a number of samples.
Furthermore, a conventional exhaust device has a simple structure having the exhaust pipe 11 connected to the side wall of the gas processing chamber (such a structure will be hereinafter referred to as “sidedraft exhaust structure”). In this conventional exhaust device, an exhaust port is normally formed on the side wall, causing much unevenness in exhaust gas flow. In order to solve such a problem, a structure having two or more connecting ports on the side wall of the gas processing chamber has been suggested, as disclosed in Japanese Laid-Open Patent Application No. 8-45917. With such a structure, however, the unevenness of exhaust gas flow can be corrected to some degree, but there is another problem that the structure becomes complicated, resulting in higher production costs.
As disclosed in Japanese Laid-Open Patent Application No. 63-111621, an exhaust device that exhausts from below the wafer stand in the coaxial direction with the central axis of the wafer has been suggested. With such an exhaust device, there are problems that the configuration of the exhaust device becomes too long in the central axis direction, and that the arrangement of various components (such as the wiring arrangement for a pusher pin for moving up and down the wafer, a suscepter heater, and a temperature heater) that are normally arranged below the wafer stand becomes difficult.
Also, in the conventional exhaust device using a porous baffle plate in a gas process chamber having a sidedraft exhaust structure, it has been difficult to designing and produce a uniform-exhaust baffle plate that can obtain uniform distribution of the flow rate of an exhaust gas in the circumferential direction on the wafer. The biggest reason of this is that a quantitative design method which is based on hydrodynamic theories was unknown. More specifically, as the baffle pore size becomes smaller, the exhaust flow rate becomes more uniform. However, the chamber inner pressure increases at the same time. As a result, this technique is not applicable to a normal process in which the chamber inner pressure needs to be low.
Meanwhile, Japanese Laid-Open Patent Application No. 8-64489 discloses a structure in which the baffle holes are arranged at varied intervals, and Japanese Patent No. 2927211 discloses a baffle plate provided with exhaust conductance adjusting holes that are displaced. However, it has been considered difficult to obtain an exhaust device that has a function to maintain uniform exhaust regardless of various process conditions such as the gas flow rate, the type of gas, temperatures and pressures. Even if the diameters and intervals of the baffle holes are varied, it is difficult to maintain predetermined uniformity in exhaust, except under specially prescribed process conditions.
Japanese Laid-Open Patent Application No. 62-98727 reads that “As shown in FIG. 2, a plurality of exhaust holes 10 are designed in accordance with the fluid conductance calculated from the flowing direction of an etching gas flowing from a gas introduction path 4 uniformly downward along the surface of the wafer 6”. However, this reference only suggests that the pore size should be larger as the location of the hole becomes more distant from the main exhaust hole, while mentioning no detailed method or technique for designing the exhaust holes.